正定矩阵里的“定”是指什么定? 正定二次型里“二次型”又是什么?

2025-03-14 11:39:02
推荐回答(3个)
回答1:

正定二次型的判别方法:
a):二次型标准形中n个系数都大于零,则其为正定;
b):二次型的对称矩阵A的n个特征值大于零,则其为正定;
c):对称矩阵A的各阶顺序主子式全大于零,则其为正定. 注:设A为n阶方阵,则位于A的左上角的1阶,2阶,...,n阶子式, 即:称为A的各阶顺序主子式.
例1:判别二次型的正定性.
解:方法一:利用二次型的对称矩阵的特征值来判断. 先写出二次型的矩阵: 由于: 可得其全部特征值:>0,>0,>0 故此二次型为正定二次型.
方法二:利用二次矩阵的各阶顺序主子式来判定. 由于此二次型的矩阵为: 因为它的个阶顺序主子式:>0,>0,>0 故此二次型为正定二次型. 除了正定二次型外,还有其他类型的二次型。 定义:设有实二次型,如果对于任意一组不全为零的实数,都有f(x)<0,则称此二次型为负定二次型,对称矩阵A称为负定矩阵;如果都有f(x)≥0,则称此二次型为半正定二次型,并称其矩阵为半正定矩阵;如果都有f(x)≤0,则称此二次型为半负定二次型,并称其矩阵为半负定矩阵。

回答2:

二次型是一个N元二次齐次多项式:正定是指当这个多项式的自变量不全为零时,多项式的值恒为正。

回答3:

问题专业,回答不了

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